Separation of water from other substances is of great utility in a wide variety of contexts.
The separation of water vapour from other gases can be achieved using hydrophilic membranes, which allow water to pass through but not the gas which is being purified.
‘Raw’ natural gas contains a number of impurities, including water vapour which must be removed before it can be used. Likewise, naturally occurring helium gas often contains water vapour which must be removed for many applications. Crude oil also frequently needs to be dehydrated as part of the refining process.
Water vapour can interfere with gas phase detector systems and pre-separation of water from the gaseous mixture can improve the sensitivity and longevity of such systems. The removal of water vapour from these and other gaseous systems may be advantageous and is an aim of the invention.
Many industries use the removal of water as a way of concentrating mixtures. Thus, the food industry will concentrate fruit juices, sugar, and milk (e.g. before cheese production) and many other industries (e.g. the galvanic industry) will concentrate their waste products in order to reduce the cost of transport and/or disposal.
Pervaporation is a method of separating mixtures of liquids using a membrane. Pervaporation consists of two basic steps: permeation of the permeate through the membrane and evaporation of the permeate from the other side of the membrane. Pervaporation is a mild process. Thus, it can be used to separate components which would not survive the comparatively harsh conditions needed for distillation (high temp, and/or low pressure). It is a low-energy alternative to distillation.
Pervaporation can be used to remove trace quantities of an impurity from a liquid. For example, hydrophilic membranes can be used to remove water from organic solvents, and can provide the solvent in a higher purity than conventional means. One particular use is to purify compounds which form azeotropic mixtures with water, such as ethanol and isopropanol.
Hydrophilic membranes can be used for the removal of water from fermentation broths or from condensation reactions such as esterification.
The currently most preferred hydrophillic membranes for pervaporation are zeolites such as those based on zeolite A. These materials have a number of drawbacks. As zeolites are polycrystalline, their formation into a stable membrane can be problematic. Zeolites dissolve in relatively weakly acidic conditions (below about pH 4) and this can limit the possible applications. It is also hard to find an effective support for such membranes.
Polymeric membranes are also widely used, but they are generally less-selective.
Graphene is believed to be impermeable to all gases and liquids. Submicrometer thick membranes made from graphene oxide are impermeable to most liquids, vapours and gases, including helium. However, an academic study has shown that, surprisingly, graphene oxide membranes which are effectively composed of graphene oxide having a thickness around 1 μm are permeable to water even though they are impermeable to helium. These graphene oxide sheets allow unimpeaded permeation of water (1010 times faster than He) (Nair et al. Science, 2012, 335, 442-444). The paper does not, however, disclose any workable membranes made from graphene oxide sheets nor does it disclose any practical applications of this material as a membrane.
The present invention aims to provide a means for dehydration (i.e. the separation of water or water vapour from a system) which is effective at a wider range of conditions than existing technologies. The system from which water is to be separated may be a mixture of gases or it may be a mixture of one or more liquids contaminated by water (e.g. a fuel system).
The present invention also aims to provide a means for dehydration which is as selective or more selective than existing technologies.
The present invention also aims to provide a means for dehydration which uses a material that is cheaper and/or easier to prepare in bulk than the materials of existing technologies.
The present invention also aims to provide a means for dehydration which is more efficient, in terms of yield or in terms of energy input than existing technologies.
The present invention also aims to provide a means for dehydration which is durable and has good longevity. This allows the material to be reused and/or recycled. This is particularly relevant for industrial uses.
The present invention satisfies some or all of the above aims.
We have found that sheets of graphene oxide, when combined with a porous support, can be used to selectively separate water or water vapour from fluid systems such as a gas, a liquid or mixtures of gases and liquids.